Bohol earthquake strongest to hit Visayas and Mindanao in over 20 years

MANILA—The energy released by the magnitude-7.2 earthquake that rocked the Visayas and Mindanao Tuesday morning was equivalent “32 Hiroshima bombs,” the country’s top seismologist said.

Director Renato Solidum of the Philippine Institute of Volcanology and Seismology (Phivolcs) told the Philippine Daily Inquirer that the earthquake was the strongest felt in the Visayas and Mindanao in 23 years.

Solidum said that the movement of the East Bohol fault triggered the tremor, which occurred at 8:12 a.m. with its epicenter located two kilometers southeast of the town of Carmen in Bohol. It occurred 33 kilometers below the surface of the earth, he said.

The temblor was felt at Intensity VII in Tagbilaran City; Intensity VI in Hinigaran, Negros Occidental, Mactan in Cebu, as well as Cebu, City Dumaguete and Mandaue City; Intensity V in Iloilo, Ozamis, Cagayan de Oro, and La Carlota cities, Guimaras Island, Abuyog town in Leyte, Sibulan town in Negros Occidental, and Gingoog town in Misamis Oriental.

The earthquake was felt at Intensity IV in Roxas, Masbate, Dipolog, Bacolod, Bayawan, Butuan and Tacloban cities; Bulusan town in Sorsogon; Hinunangan in Leyte; Patnungon in Antique; Tabon in Leyte; Baybay in Southern Leyte; San Pablo in Leyte; Bato, Leyte; San Jose, Antique; and Guihulngan, Negros Oriental.

It was felt at Intensity III in Davao, Canlaon, Cotabato, Zamboanga, Surigao and Borongan Cities; as well as La Costellana town in Negros Occidental; Bukidnon Province; Tarragona town in Davao; Laoan in Antique; Banisilan, North Cotabato; and the towns of Potongan, Polanco, Labason, and Manukan, all in Zamboanga del Norte.

The quake was felt at Intensity II in Tacurong City; Nabas and Libertad towns in Aklan; as well as Carrascal, Surigao del Sur. It was felt at Intensity I in Lezo town in Aklan.

Magnitude is a measure of the energy released by the earthquake while intensity measures the effects of an earthquake on objects or the environment as felt or perceived by humans.

According to Phivolcs, an Intensity VII earthquake is considered destructive. “Most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly-built structures suffer considerable damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surface, or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly.”

It defines liquefaction as a process whereby loose saturated sand loses strength during an earthquake and behaves like liquid.

An earthquake with a magnitude of 7 or 8 is considered major and can cause considerable damage near the epicenter. Shallow-seated or near-surface major earthquakes when they occur under the sea, may generate tsunamis. Tuesday morning’s quake did not trigger a tsunami.

Solidum told the Philippine Daily Inquirer that the last strong earthquake recorded in Bohol was on February 8, 1990. It had a magnitude of 6.8 and its epicenter was located 17 kilometers east of Tagbilaran City.

According to Phivolcs records, the quake caused “ground fissures, landslides, rockfalls, ground subsidence and collapse, sand or mud fountaining and sudden increase on the sea level. Most of the manifestations were particularly observed and experienced by the towns of Jagna, Valencia, Duero, Guindulman and Garcia Hernandez. The force of the incoming waves from the sea caused Alijuan River in Duero to flow inland immediately after the earthquake.”

The strongest recorded earthquake to hit Luzon had a magnitude 7.9 and struck on July 16, 1990, with its epicenter near Rizal town in Nueva Ecija. An estimated 1,600 people were killed in the temblor. That quake devastated Baguio City and caused extensive damage in Pangasinan and Nueva Ecija.

Asked how he would describe the strength of Tuesday’s earthquake, he said, “The magnitude, the energy released by the quake is equivalent to 32 Hiroshima bombs.” Solidum added that he expected more aftershocks to follow the major tremor.

As of noon Tuesday, Phivolcs recorded 137 aftershocks in Carmen and Tagbilaran towns in Bohol the strongest of which had a magnitude of 4.3 and occurred at 9:37 a.m. It was felt at Intensity IV in Tagbilaran City.

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